This section is from the book "A Dictionary Of Modern Gardening", by George William Johnson, David Landreth. Also available from Amazon: The Winter Harvest Handbook: Year Round Vegetable Production Using Deep Organic Techniques and Unheated Greenhouses.
The proper time is at the end of July, not later. Shorten the main or tap-root, as it is called, to within about half an inch of the leaves, that a few of the vigorous young fibrous roots only may be retained; with a small trowel make a hole in the soil sufficiently deep that the plant may have the very crown of the main root covered at least one inch with the soil.
"It is of the utmost importance that the polyanthus should be thus deep set; for the young roots always emanate from the very top of the main root, and throw themselves out for the most part, if a somewhat curved or horizontal method of planting is adopted.
"Having well watered the bed, the plants require no more care whatever, except being kept clean. They should be left entirely without covering or protection in the winter.
In spring, just when the pips are about to expand, if wanted for the purpose of exhibition, or to ornament the stage or cool greenhouse, they may he taken up with a moderately-sized ball of earth, and put into common auricula pots, for they bear this removal without the slightest injury or apparent check. Water must now be supplied rather freely, and should occasionally be sprinkled over the foliage (avoiding the flowers); and the smaller and central pips should be thinned out, that the truss or umbel of Mowers may have a uniform and un-crowded appearance. When the bloom is over, the plants should be turned out of their pots into the border from whence they were taken; here they must remain without further care, except an occasional watering, till the end of July, when they should be taken up, the soil crumbled from their roots, and those which present two or more heads divided gently with the fingers, and prepared and planted as before described." - Gard. Chron.
 
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